The issue is likely the opto emitter, or the resistors on the trough board have legs that have broken off.

When I got my STTNG, someone had 'repaired' the trough by adding additional globs of solder to the broken resistor legs.

I ordered a metric crapton of resistors for the ball trough board when I did my repair, let me know if you'd like me to drop a few in the mail for you. If you're after a more plug & play solution, Great Lakes Modular sells replacement transmitter & receiver boards, for not very much moiney.

http://www.greatlakesmodular.com/index.html?http://www.greatlakesmodular.com/products/pinball/dcs_tr.html

Yes, if the problem is the opto, usually it is the emitter -- the product you linked to -- that is the problem. You can try cleaning it (as Jediturtle says - cotton swab dipped in glass cleaner works great) along with the paired receiver.

If that doesn't help, you may need to replace them. Like I said, usually the emitter is the problem, but you may want to replace the pair while you are at it:

http://www.pinballcenter.eu/catalog/opto-infrared-diodes-leds-pair-p-252.html?language=english

It's not a difficult job if you are comfortable soldering.

It may be worth checking the resistors on the trough boards as well (just check that the legs are not broken).